1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, and in particular to a semiconductor device having an element isolating oxide film and a method of manufacturing the same.
2. Description of the Background Art
A LOCOS (Local Oxidation of Silicon) method has been known as a conventional method of forming element isolating regions in VLSIs. Referring to FIGS. 24-26, a conventional LOCOS method will be described below. First, as shown in FIG. 24, a silicon oxide film (SiO.sub.2 film) 2 having a thickness from about 300 to about 500 .ANG. is formed on a silicon substrate 1 of, e.g., P-type. A silicon nitride film (Si.sub.3 N.sub.4 film) 3 having a thickness from about 500 to about 1000 .ANG. and forming a anti-oxidation film is formed at a predetermined region on silicon oxide film 2. Using silicon nitride film 3 as a mask, thermal oxidation is performed to form a field oxide film (element isolating oxide film) 4 having a large thickness as shown in FIG. 25. Then, nitride film 3 is removed by etching, and oxide film 2 is removed, so that a configuration shown in FIG. 26 is formed.
However, the oxidation for forming field oxide film 4 progresses not only in a vertical direction with respect to silicon substrate 1 but also in a parallel direction. This is due to the fact that silicon oxide film 2 not having sufficient anti-oxidation properties is used as a base film under silicon nitride film 3. Due to employment of silicon oxide film 2, a so-called bird's beak 4a is formed at an end of field oxide film 4, which impedes densification of elements.
A B/B length (see FIG. 25) which is a length of bird's beak 4a can be represented with a distance to an end of nitride film 3 from a point at which the thickness of oxide film 2 starts to vary. The B/B length is substantially proportional to the film thickness of field oxide film 4. It is desirable to minimize the B/B length for densification of the device. For example, if a structure is to be miniaturized to have an active region (i.e., silicon nitride film 3) of about 1 .mu.m or less in width, the B/B length must be from about 0.15 to about 0.10 .mu.m. In order to achieve the B/B length from about 0.15 to about 0.10 .mu.m, however, the thickness of field oxide film 4 must be from about 1000 to about 1500 .ANG.. However, such a small thickness of field oxide film 4 impairs electrical isolating properties.
In the prior art, as described above, reduction of the B/B length disadvantageously impairs the isolating properties of field oxide film 4. Consequently, it is difficult to reduce sufficiently the B/B length while maintaining sufficient isolating properties.
In the conventional filed oxide film 4 shown in FIG. 26, the following problem arises in connection with flatness of its upper surface. .theta.i and t.sub.U are parameters representing the upper flatness of field oxide film 4. Referring to FIGS. 27 and 28, .theta.i and t.sub.U will be described below. A structure shown in FIG. 27 is formed in such a manner that a gate oxide film 5 is formed after the step shown in FIG. 26, and further a polycrystalline silicon layer 6, which will form a gate electrode, is formed by a low pressure CVD method. FIG. 28 is a perspective view of the structure shown in FIG. 26.
Referring to FIGS. 27 and 28, t.sub.U represents a thickness or distance from a base, which is an upper surface gate oxide film 5, to an upper surface of a thickest portion of field oxide film 4, and t.sub.OX and t.sub.G represent film thicknesses of gate oxide film 5 and polycrystalline silicon layer 6, respectively. .theta.i represents an angle defined between upper surface 51 of gate oxide film 5 and a tangent 401 at a given point 402 in an area between a point at which film thickness t.sub.OX of gate oxide film 5 starts to increase and a point at which field oxide film 4 has the largest thickness.
Film thickness t.sub.XG of polycrystalline silicon layer 6 located at the bird's beak of field oxide film 4 satisfies a relationship of t.sub.XG =t.sub.G /cos .theta.i. Therefore, the film thicknesses satisfy a relationship of t.sub.XG &gt;t.sub.G. When patterning gate oxide film 5 and polycrystalline silicon layer 6 for forming the gate electrode, polycrystalline silicon layer 6 having thickness of t.sub.G is to be removed at the active region, while polycrystalline silicon layer 6 having thickness of t.sub.XG is to be removed at the bird's beak. Thus, the active region is excessively etched. In this case, if a selection ratio of polycrystalline silicon layer 6 with respect to gate oxide film 5 is small and gate oxide film 5 is thin, such a disadvantage occurs that gate oxide film 5 at the active region is shaved. This results in a problem that the surface of semiconductor substrate 1 is exposed and shaved. This adversely affect the device.
Reduction of the thickness of gate oxide film 5 is inevitably required for reducing the power supply voltage of the semiconductor device. Also, it is difficult to increase a selection ratio. Therefore, it is required to provide field oxide film 4 of a flat structure in which .theta.i and t.sub.U described above are minimized. According to the conventional manufacturing process shown in FIGS. 24 to 26, however, it is difficult to form field oxide film 4 having reduced .theta.i and t.sub.U and thus having good upper flatness. Accordingly, the surface of semiconductor substrate 1 is shaved at the step of etching the polycrystalline silicon layer 6 forming the gate electrode, and thus the device is adversely affected as already described.
In the prior art, as described before, it is difficult to reduce the length of the bird's beak while maintaining intended isolating properties, because silicon oxide film 2 which is susceptible to oxidation is used as the base film under silicon oxynitride film 3. Also, stable processing of the gate electrode is difficult because it is difficult to improve the upper flatness .theta.i and t.sub.U) of field oxide film 4.
Meanwhile, a polybuffer LOCOS method has been known as a method by which the bird's beak length (B/B length) can be reduced while preventing reduction of the isolating properties of field oxide film 4. According to this polybuffer LOCOS method, an oxide film is formed on a semiconductor substrate, and a polycrystalline silicon layer is formed on the oxide film. A nitride film is formed at a predetermined region on an upper surface of the polycrystalline silicon layer. According to this method, the polycrystalline silicon layer relieves a stress which is generated when forming the field oxide film, so that the thickness of the nitride film can be increased. Consequently, the bird's beak length can be reduced.
According to this method, however, field oxide film 4 includes a negative angle portion 10 of a configuration shown in FIG. 29. Negative angle portion 10 is a portion at which .theta.i is 90.degree. or more.
Negative angle portion 10 causes such a disadvantage that an unetched portion remains at the negative angle portion 10 after the etching step for patterning the polycrystalline silicon layer 6. This may cause a short circuit of the gate electrode. A process of generation of negative angle portion 10 will be described below with reference to FIGS. 30-41. FIGS. 30-41 show simulated process of generation of negative angle portion 10.
As shown in FIG. 30, silicon oxide film 2 having a thickness from about 300 to about 500 .ANG. is formed on, e.g., P-type silicon substrate 1, and a polycrystalline silicon layer 7 having a thickness from about 500 to about 1000 .ANG. is formed thereon. Silicon nitride film 3 having a thickness from about 1000 to 2000 .ANG. is selectively formed at a predetermined region on polycrystalline silicon layer 7. Using this silicon nitride film 3 as a mask, thermal oxidation is performed, in which case variation with time occurs as shown in FIGS. 30 through 41.
Referring to FIGS. 30 to 41, it can be understood that negative angle portion 10 is generated due to oxidation of polycrystalline silicon layer 7 located at a side end of silicon nitride film 3. After the step shown in FIG. 41, nitride film 3, polycrystalline silicon layer 7 and silicon oxide film 2 are removed, and then polycrystalline silicon layer 6 forming the gate electrode is formed as shown in FIG. 29. In the conventional polybuffer LOCOS method, as described above, the polycrystalline silicon layer 7 is used as a buffer film, so that negative angle portion 10 is formed, which results in the problem that short-circuit of the gate electrode may occur.
As another method of reducing the bird's beak length (B/B length), such a method has been proposed that LOCOS oxidation is performed with a two-layer structure formed of a silicon oxynitride (SiO.sub.x N.sub.y) film and a silicon nitride film (Si.sub.3 N.sub.4 film). This is disclosed in "1987 VLSI Symposium", pp. 19-20. According to this method, however, a white ribbon 12 made of nitride is formed on the substrate surface as shown in FIG. 42. The cause of generation of white ribbon 12 will be described below. During oxidation process shown in FIG. 42, reaction represented by the following formula (1) occurs between Si.sub.3 N.sub.4 and water contained in oxidation atmosphere at the surface of silicon nitride film 3 at the end of field oxide film 4. EQU Si.sub.3 N.sub.4 +H.sub.2 O.fwdarw.SiO.sub.2 +NH.sub.3 (1)
Thereby, ammonia (NH.sub.3) is generated, and moves through field oxide film 4 to the silicon substrate surface under silicon oxynitride (SiO.sub.x N.sub.y) film 31 located under silicon nitride film 3. At the silicon substrate surface, the ammonia reacts with silicon to produce nitride, i.e., white ribbon 12. In this case, since white ribbon 12 is covered with silicon oxynitride film 31, it is not removed by the etching effected for removing silicon nitride film 3. Also, white ribbon 12 is not removed by the etching effected for removing silicon oxynitride film 31.
Accordingly, a problem occurs at the later step of forming the gate oxide film on the silicon substrate surface, and specifically a stable gate oxide film cannot be formed because white ribbon 12 impedes the oxidation. FIG. 43 shows a structure after removal of silicon nitride film 3 and silicon oxynitride film 31. According to the conventional method of performing LOCOS oxidation with the two-layer structure formed of silicon oxynitride film 31 and silicon nitride film 3 as described above, white ribbon 12 is formed, which results in a problem that the gate oxide film of MOSFET cannot be formed uniformly.
According to "1987 VLSI Symposium", pp. 19-20 described before, such a method is employed that etchback is effected after the silicon oxide film is formed on the entire surface of the field oxide film in order to improve the upper flatness of the field oxide film. According to this method, such a disadvantage occurs that the end of the field oxide film is hollowed deep at an etchback step. The reason for this is as follows. Since a stress concentrates at the end of the field oxide film, the end is etched more rapidly than the other portions. Therefore, etching of the end of the field oxide film progresses more rapidly that the other portions during the etchback, resulting in the deep hollow. As a result, it is actually difficult to improve the upper flatness of the field oxide film.
Although various methods have been proposed for reducing the bird's beak length and improving the upper flatness of the field oxide film as described above, the foregoing problems arise in these methods.